Pv-optimiser power system for supply of power from a photovoltaic installation

ABSTRACT

A PV-optimiser power system for a photovoltaic installation for supply of power from a photovoltaic installation. The system includes a first DC/DC converter connected to a PV panel and to one or more energy storage modules, and a second DC/DC converter, connected in parallel to the PV panel in a string of PV panels of a PV installation, wherein the second DC/DC converter is configured to operate as an optimiser and execute a maximum power point tracking algorithm (MPPT) to determine the maximum power output of the PV panel of the plurality of the PV panels in the string.

TECHNICAL FIELD

The present invention discloses a PV-optimiser power system for supplyof power from a photovoltaic installation. In particular, the inventionrelates to an auxiliary power system to supply electrical power to solartrackers, electronic control, security systems, monitoring systems andothers electronic equipment close to the devices that require electricalpower.

Photovoltaic (PV) installations usually include secondary orcomplementary systems or devices which requires an electrical powersource in order to function. Several solutions commonly used in thecurrent state of the art include:

-   -   1. Low voltage electric installations powered by an electrical        grid, which requires the use of a second electric installation        increasing costs    -   2. Low voltage electric installations powered by PV        installation, which also requires the use of a second electric        installation increasing costs    -   3. High voltage power supply from strings of the PV        installation, which requires the use of a high voltage electric        installation and a high voltage DC/DC converter increasing costs        and complexity.

The term “photovoltaic installation”, or PV installation should beunderstood herein as an alternative to solar photovoltaic plant,photovoltaic power station, solar park, solar farm. photovoltaicinstallation or photovoltaic system, all of which are known used termsin this specific field.

Furthermore, the abbreviation/acronym “PV” may be used throughout thisdocument instead of the term “photovoltaic”.

In this description a PV-optimiser power system will be understood as aDC to DC converter technology implemented to maximize the energyharvested from a solar photovoltaic panel.

STATE OF THE ART

WO2017174829 discloses an installation comprising: an arrangement forgenerating a direct current, formed by electrical generators (PV1 . . .PVn) which are connected in series and located inside of a local zoneand which supply to a remote zone a total direct current that is the sumof the current generated by each of the electrical generators (PV1 . . .PVn); an auxiliary power supply device (D) disposed inside of the localzone and which provides local supply voltage to an auxiliary device (E),the auxiliary power supply device (D) being composed of a DC powerconverter (CP) electrically connected in series by respective inputterminals (T1, T2) in the arrangement for generating direct currentbetween two connection points (p1, p2) of the electrical generatorslocated inside of the local zone

US2028/0115165A1 discloses a rechargeable battery controller combinedwith a rechargeable battery and used in an existing PV system. Thecontroller includes a DC-DC converter. which allows power to be passedbetween a power line and a rechargeable battery, and a control unit,which determines whether maximum power point tracking (MPPT) controlusing hill climbing is being performed by a PCS based on an inputvoltage or current value of the PCS. The control unit regulatescharge/discharge power of the rechargeable battery to allow input powerof the PCS to be a target value based on the input voltage and currentvalues of the PCS while MPPT control using hill climbing is performed,and maintains, in a period during which MPPT control using hill climbingis not performed, the charge/discharge power to be the power at abeginning of the period.

However solutions in which a DC/DC converter to feed a charge orconsumer is connected in series to one or more PV panels of a PV stringof the PV installation, have proven to be inefficient because they cancause a voltage drop at string terminals if any one of the PV panels insaid string having connected a DC/DC converter is malfunctioning throughthe use of bypass diodes, reducing the power outcome of the PVinstallation and causing a voltage drop at string terminals.

BRIEF DESCRIPTION OF THE INVENTION

The invention proposed herein is applicable to a PV installation havinga several strings of PV panels and with at least one DC/DC converterconnected to one PV panel of said plurality of PV panels of said string,to feed one or more energy storage modules or consumers as in theprevious cited patent documents. This first DC/DC converter isconfigured to redirect a portion of power generated by said PV panel tothe cited one or more energy storage modules.

The strings of PV panels of the PV installation are connected to acentral inverter of the PV installation.

The invention proposes the use of a second DC/DC converter, connected inparallel to said PV panel of said string (including cited first DC/DCconverter) in a way that avoids the entry of the by-pass diode of the PVpanel into conduction and at the same time allows the maximum possibleenergy to be extracted from the panel to which the consumer has beenconnected. According to this solution said second DC/DC converter isconfigured to operate as an optimiser and execute a MPPT algorithm todetermine maximum power output of the PV panel, so that indirectlyassisting the central inverter to obtain a maximum power of said atleast one string. The output current of the second DC/DC converter isdetermined by the string.

Moreover, for a good operation of the PV-optimiser power system thesecond DC/DC converter is configured to operate at a voltage over thanthe first DC/DC converter minimum starting load voltage VSL needed forsaid first DC/DC converter to be started.

Other features of the invention appear from the following detaileddescription of an embodiment.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and other advantages and features will be more fullyunderstood from the following detailed description of an embodiment withreference to the accompanying drawings, to be taken in an illustrativeand not imitative, in which:

FIG. 1 is a block diagram of the PV-optimiser power system for supply ofpower from a PV installation, showing the second converter aimed toimplement a MPPT algorithm, connected in parallel with the firstconverter, said second converter also called optimiser in that it avoidsthe conduction of the bypass diodes at the same time that allow toobtain a maximum energy from the PV panel. It should be emphasized thatPV panel 4 b, does not form part of the power system but of a stringwhere the power system is installed. This has been indicated byillustrating the PV panel 4 b outside a rectangle representing thePV-optimiser power system.

FIG. 2 is a schematic representation of the functional principle of theproposed PV-optimiser power system with indication of the centralinverter.

FIG. 3 is a simplified representation of the PV panel voltage-currentcharacteristic curve, load curve, panel+ load curve and power panel+load curve.

FIG. 4 is a representation of an PV-optimiser power system according tothis invention within a string and with several additional strings ofthe whole set of strings feeding the input voltage of a centralinverter.

DETAILED DESCRIPTION OF AN EMBODIMENT

The foregoing and other advantages and features will be more fullyunderstood from the following detailed description of an embodiment withreference to the accompanying drawings, to be taken in an illustrativeand non-(imitative manner, in which:

FIG. 1 represents a block diagram of the PV-optimiser power system 5 ofthis invention. This PV-optimiser power system 5 is connected in serieswith the existing string.

It shows a first DC/DC converter 1 connected in parallel to one PV panel4 b of the string. This first DC/DC converter 1 is configured toredirect a portion of power generated by the PV panel 4 b to one or moreenergy storage modules 3 (for example a battery). The portion of energyredirected by the DC/DC converter 1 is lower than the total energygenerated by the PV panel 4 b.

FIG. 1 also includes according to the principles of this invention asecond DC/DC converter 2, connected in parallel to the PV panel 4 b andthis second DC/DC converter is configured to operate as an optimiser andto execute a MPPT algorithm to determine the maximum power output of thePV panel (4 b), so that indirectly assisting the central inverter (6) toobtain a maximum power of said at least one string (including thissecond DC/DC converter 2). Therefore, the output current of the secondDC/DC converter 2 is determined by the string and the MPPT algorithm ofthe second DC/DC converter 2 controls the output of the second DC/DCconverter 2 voltage delivered to the string. The energy storage modulescharger 1 controls the current supplied to the energy storage modules 3that comes from the PV panel 4 b according to the working pointcontrolled by the second DC/DC converter 2. The energy storage modules 3accumulate the current supplied by the PV panel 4 b controlled by theenergy storage modules charger 1.

According to the principles of this invention a voltage input of thesecond DC/DC converter 2 is equal to the voltage of the PV panel (4 b)adjusted by the MPPT algorithm of the second DC/DC converter 2.

According to a preferred embodiment of the invention, the second DC/DCconverter (2) is a converter that reduces voltage from its input to itsoutput. Several different solutions can be implemented, among them theuse of a buck converter, or two or more converters in parallel, etc.

FIG. 2 is a simplified scheme for ease of explanation representing thefunctional principle of the PV-optimiser power system 5 of thisinvention. The voltage from the strings 11 is feeding a central inverter6 of the installation. A central inverter is a DC/AC power inverterconnected to the electrical grid. This central inverter 6 regulates thevoltage according to the MPPT of whole installation. This regulationdone by the central inverter 6 is a slow voltage change to avoid currentharmonic distortion. Therefore, the MPPT algorithm of the second DC/DCconverter 2 of the PV-optimiser power system 5 has to have a fasterresponse than the MPPT algorithm of the central inverter 6, which meansthat the PV-optimiser power system does not affect the performance ofthe string in which it is installed, and also doesn't affect theperformance of the set of strings of the entire installation.

Considering in greater detail this FIG. 2 , the V_(inv) voltagerepresents the input voltage of the central inverter 6. The centralinverter 6 will modify this voltage to find the maximum power point ofthe installation. Since the central inverter 6 is injecting energy intothe grid and needs to maintain a harmonic distortion of the low current,this voltage will not change quickly. This point is important because itis assumed at all times that the search for the maximum power point ofthe optimizer (converter 2) is much faster than that of the centralinverter 6. In today's central inverters this approach is fully valid.The string in the embodiment of this FIG. 2 consists of 3 panels forsimplicity, since the number of panels does not affect the approach ofthe problem. The I_(c) current represents the consumption of the loadconnected to the panel. This current can be calculated as the powerrequired by the load divided by the voltage of the panel at any givenmoment. A current source has been assumed as the DC/DC converter mustmaintain the value of the voltage applied to the panel constant andproperly filtered.

If the algorithm of the maximum power point tracker of the centralinverter 6 operates properly, the I_(s) current (input current to thecentral inverter 6) must be very close to the current of the maximumpower point of the panels (I_(s)=I_(mpp)). Assuming that all panels areequal and that they are receiving the same radiation, they must all beworking under these conditions with a voltage approximate to theirmaximum power point voltage V_(mpp) (VP3=VP2=VP1=V_(mpp)). Assuming a100% efficiency in the converter, the power delivered to the string isequal to V_(mpp)*I_(mpp)−V_(mpp)*I_(c), i.e. the power provided by thepanel minus the load. If the rest of the installation is also at itsmaximum power point, the current I_(s) is equal to I_(mpp), so the powerat the converter output can be calculated as indicated in equation (1).Under these conditions it can be deduced that for the system to workcorrectly the output voltage must be lower than that of the maximumpower point, as shown in equation (2).

V _(o) ·I _(mpp) =V _(mpp) ·I _(mpp) −V _(mpp) ·I _(c)  (1)

V _(o) =V _(mpp) −V _(mpp) ·I _(d) /I _(mpp)  (2)

According to these results and under the premise that galvanic isolationis not necessary in this part, the converter 2 to be used can be areducing converter or possibly two converters connected in parallel with180° out of phase ignition, to reduce the input capacitor and the outputfilter coil. The efficiency of these converters can be very high, closeto 99%.

FIG. 3 schematically represents a PV panel voltage-currentcharacteristic curve showing a simple load profile and a constantcurrent source that is activated from the presence of a minimum startingload voltage VSL 9. As the load (converters 1 and 2) and the PV panel 4b are electrically connected in parallel, the current is the sum of thetwo currents. In this case the load would have a negative sign notincluded in the drawing but represented in the resulting characteristiccurve, that shows a relative maximum 7 at the time the load is connected(VSL). In order for the search algorithm for the maximum power pointtracking to work correctly, the absolute maximum point 8 should be takenand not the relative maximum 7 that is caused when the load isconnected, and for this reason the value must be greater than the VSLvalue 9.

FIG. 4 is a representation of the PV-optirniser power system installedin the string of the photovoltaic installation. The voltage from strings11 is the voltage input of the central inverter 6 that will connect tothe electrical grid. The figure shows an embodiment where thePV-optimiser power system 5 including the PV panel 4 b with theconverters 1 and 2 in parallel, is connected in series between PV panel4 a and 4 c of a string.

The algorithm controlling the first DC/DC converter 1 further takes intoaccount the temperature and technology of the energy storage modules 3for optimizing the redirected power when charging the energy storagemodules 3.

The electrical isolation between the input and the output of the secondDC/DC converter 2 is of at least 4 KV.

According to a further embodiment of the invention the proposedPV-optimiser power system 5 is configured to automatically adjusts theload injected by first DC/DC converter 1 to the energy storage modules 3according to the radiation impinging on the PV panels of at least onestring of a solar tracker, so that the load injected to the energystorage modules 3 is increased according to the solar radiation power.

For this aim means such as sensors, control and actuators can beincluded to automatically adjust the DC/DC converter 1 consumptionaccording to the radiation impinging on the PV panels of said at leastone string of a solar tracker. The aim is to minimize losses in thestring and apply a more intense loading of the energy storage modules 3when there is more photovoltaic power.

The inventors have found that is important to be able to regulate theconsumption of the converter 1 according to the radiation that impingesonto the solar panels or in an alternative way according to the stringcurrent.

The higher the ratio between the power of the panel and the power of theload 3 to be fed, the lower is the effect on the whole installation.

These measures allow to take advantage of the moments of greaterradiation of the day to perform the feeding of the load 3 under a greatpower exigency.

It is also possible to avoid connecting the load early in the morningand late in the afternoon.

In case of very low radiation, the load should be adjusted to the powergenerated by the panel.

It will be understood that various parts of one embodiment of theinvention can be freely combined with parts described in otherembodiments, even being said combination not explicitly described,provided there is no harm in such combination.

1.-10. (canceled)
 11. A PV-optimiser power system for supply of powerfrom a photovoltaic installation, in which at least a string of a solartracker of the PV installation comprises a plurality of connected PVpanels, connected to a central inverter of the PV installation, thepower system comprises: at least a first DC/DC converter, and one ormore energy storage modules charged by the first DC/DC converter;wherein the at least one first DC/DC converter is connected to one PVpanel of the plurality of PV panels of the string; wherein thePV-optimiser power system further comprises: a second DC/DC converter,connected in parallel to the PV panel of the string, wherein the secondDC/DC converter is configured to operate as an optimiser and to executea maximum power point tracking, MPPT, algorithm to determine maximumpower output of the PV panel, so that indirectly assisting the centralinverter to obtain a maximum power of the at least one string; whereinthe first DC/DC converter, also connected in parallel to the PV panel ofthe string, is configured to redirect a portion of power generated bythe PV panel to the one or more energy storage modules; wherein theoutput current of the second DC/DC converter is determined by thestring; and wherein the PV-optimiser power system is connected inparallel with the PV panel and in series with the plurality of PV panelsof one of the strings of the PV installation.
 12. The PV-optimiser powersystem according to claim 11, wherein a voltage input of the secondDC/DC converter is equal to the voltage of the PV panel adjusted by theMPPT algorithm of the second DC/DC converter.
 13. The PV-optimiser powersystem according to claim 11, wherein the MPPT algorithm of the secondDC/DC converter has a faster response than the MPPT algorithm of thecentral inverter.
 14. The PV-optimiser power system according to claim11, wherein the second DC/DC converter is configured to operate at avoltage over than the first DC/DC converter minimum starting loadvoltage VSL needed for the first DC/DC converter to be started. 15.PV-optimiser power system according to claim 11, wherein the secondDC/DC converter is a converter that reduces voltage from its input toits output.
 16. The PV-optimiser power system according to claim 11,wherein the input and the output of the second DC/DC converter iselectrically isolated.
 17. The PV-optimiser power system according toclaim 11, wherein the system is configured to automatically adjusts theload injected by first DC/DC converter to the energy storage modulesaccording to the radiation impinging on the PV panels of at least onestring of a solar tracker, so that the load injected to the energystorage modules is increased according to the solar radiation power.